Sorry had to do other stuff yesterday
OK I will explain how the background works.
It can be displayed in many ways but will talk about the way it is displayed now.
These are the settings i have applied:
It is displayed from the center top (image is centered on the screen and starts at the top)
Hmm thats odd, if you look closely to your screenshot then you can see the background doesn't scale, it actually just repeats itself, it actually only shows abit of it and not everything so you can in theory scale it down so it shows more logos on the same size.
It is also fixed so when you scroll, the forum moves but the background does not (this is the recommended way and looks better)
This also means if the background photo is bigger than your screen res then it will never repeat (it is only repeating in my screen cap because i took a photo of the full page, not my screen)
Larger photos look better as they don't repeat on Hi-Res screens the down side is they have a large file size and take longer to load for those with slow internet speeds.
I can reduce the quality of the photo in Photoshop and still keep the resolution but it is a fine balance between quality and load time.
I can like you say reduce the photo down so you see more on the sides but this will make it repeat on most screens, this would probably work ok with the background displayed as I post this but will not work well with a photo as the join will stand out.
Does the background need to be a specific size, or can it use anything ?
Like i said above it can be any size but is a trade off due to the load times.